feat(dictionary): add Hachidori backend support

- Add backend selection, setup gating, Anki integration, and external host support
- Add launcher flags, documentation, packaging, and focused tests
- Open on-demand overlay modals on the first attempt

vendor/hachidori/extension/overlay-mode.js

@@ -0,0 +1,31 @@
+// Hosts that embed Hachidori in an overlay, such as the GSM overlay, set this to true in their copy.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+import { BROWSER_KIND, IS_FIREFOX } from "./browser-api.js";
+
+export const OVERLAY_MODE = true;
+export const HOST_BROWSER = OVERLAY_MODE ? "electron" : BROWSER_KIND;
+
+// Electron's extension host deliberately exposes less of Chrome than a normal
+// browser window, and Firefox deliberately omits Chrome's recording stack.
+// Keep every host-owned capability in one place so shared settings cannot make
+// an unavailable control live again.
+export const HOST_CAPABILITIES = Object.freeze({
+
+  browserShortcuts: !OVERLAY_MODE,
+  // `customLinks` is the pre-Custom-buttons host contract. Keep the alias so
+  // an embedding host can update its reader before updating its capability
+  // projection.
+  customLinks: true,
+  // Chrome MV3 `userScripts` registration; Firefox MV2 has no equivalent yet.
+  customJavaScript: false,
+  linkButtons: true,
+  externalLinkHost: OVERLAY_MODE,
+  localFileAccessPrompt: !OVERLAY_MODE,
+  mediaCapture: !OVERLAY_MODE && !IS_FIREFOX,
+});
+
+export const MINING_CAPABILITIES = Object.freeze({
+  screenshot: !OVERLAY_MODE,
+  browserSpeech: !OVERLAY_MODE && !IS_FIREFOX,
+});